I think the ROI might not be quite as simple as dividing profit by deposits. For example; you deposit £1000, then you withdraw £500, then you deposit £500. Your deposits would show £1500 but you have only ever had a maximum of £1000 of 'your money' in the account. Note that this is different from depositing £1000, then depositing £500, then withdrawing £500.

I think i get that to do it properly you have to do on an individual futures basis which is quite complicated (noir's spreadsheet perhaps might help)...

But in laymans terms... keep a track of how much you put in... Keep a track of how much you take out and add that on to your current portfolio value and I think that will still give you the best reflection?? (you can add in balance, take out dividends, bonuses etc but I would try and keep it simple)...

After all... when all is finished you'll have spent so much and withdrawn so much... so the answer will lie between those on to how much profit you have or haven't made?